This invention relates to an apparatus and a method for effecting a response to the matric potential of a medium such as soil which can be useful as a measurement or as a control for water supply.
This then may include a matric potential responder, an actuator for use in a responder, a sensor for use in a responder and method of measuring matric potential.
It is known to measure some factors of soil to establish some guide to an amount of water that might exist in soil to facilitate growth of plants in that soil.
There are accordingly devices measuring the electrical resistance of the soil, the electrical impedance of the soil or even the dielectric constant of the soil, each of which measures are useful in some cases but do not however provide a measure of matric potential.
Plants derive access to moisture in soil by overcoming matric potential in the soil or as it is sometimes termed xe2x80x9csoil suctionxe2x80x9d.
Such a characteristic is currently able to be measured by a tensiometer but there are significant difficulties with any known tensiometer. One such difficulty but not the only difficulty is that a tensiometer is not useful outside of a relatively small range of matric potentials (zero kilopascals through to 80 kilopascals) which range is substantially less than would be useful to assist in assessing water availability for plants.
An object of this invention is to provide a further method and apparatus by which matric potential can be measured or used to assist in watering functions for plants which can at least provide an extended range of measurement or response and which, at relatively small cost, can provide with reasonable accuracy such response or measurement.
It is previously known to use a water swellable material which when put in contact with water will swell and then use this effect to close a valve supplying water.
Hitherto, most such proposals have described a water swellable material as a material that will, in the presence of sufficient water, swell so that given enough time and enough water the extent of swelling will be sufficient to close a valve against significant resistance or trigger a release against a substantial amount of resistance, with the trigger then further releasing some valve or other operation for control of water supply.
Such previous applications then seem to treat the use of such material as simply a detector of the presence of water so that with water present and given sufficient time and generally total water inundation this change in status of the material from not swollen to swollen or the opposite, can then be used to trigger some control either directly or indirectly.
In serious water use applications such an approach has serious deficiencies. It does not, for instance, provide for commencement of watering in conditions which are ideal for the plant where any drying beyond perhaps matric potential of 150 kilopascals is undesirable. If one cannot measure this then it is an open question as to when a start might occur and this is simply unacceptable for serious watering of plants.
Such devices that I have previously seen proposed, then, are not set up to or do not respond to take account of the matric potential of the medium to an extent that makes them useful for serious control of watering.
I have found that it is possible to measure matric potential using water swellable non-liquid materials and further more use these materials to then usefully control supply of water for serious watering purposes. This discovery results from an understanding of what seems to be a basic characteristic of such materials. If the material is exposed to a selected value of matric potential then this will effect an internal suction effect within the whole of the material. If we then apply a measured external force to the material which has the effect of applying throughout the material a compression, then soil suction effect or matric potential within the material will be offset by this compression.
In one form this invention can be said to reside in a matric potential responder which includes a liquid absorbing swellable material which will exhibit an internally developed increase or decrease in expansive pressure within the material in response to the matric potential of hydraulically connected soils or other medium, means applying and/or maintaining a compressive pressure to the said material, and means adapted to effect an output in response to changes in respect of the internal pressure of said material.
In preference, the material selected and the compressive pressure applied are such that the responder will provide an output to detected matric potential within the range of from 50 kilopascals to 300 kilopascals.
In preference the matric potential responder includes a housing at least one part of the housing being porous to provide for the effect of matric potential within liquid to be able to transfer therethrough, and within the housing, a liquid absorbing swellable material which will exhibit an internally developed increase or decrease in expansive pressure within the material in response to the matric potential of the hydraulically connected materials, means to apply compressive pressure to the said material within the housing, and means adapted to effect an output in response to changes in respect of the internal pressure of said material.
In one preferred form, there is a ratio of hydrogel to container or housing volume which will result in an internally generated pressure so that with unlimited access to water such that pressure will be 200 kilopascals. There is provided a piston with a limit to its outward movement subject to a resilient pressure such that an externally applied pressure will be 100 kilopascals to the material.
When the hydrogel is exposed to a matric potential of greater than 100 kilopascals, the piston will displace the material to an extent resulting in a change of volume and inward movement of the piston dependent upon the extent of the soil matric potential to which the hydrogel is exposed. This movement can be utilized by connecting it to other mechanisms for indicating and control purposes.
With a housing defining a substantially fixed volume, a change in matric potential of swellable material held confined by the housing will be exhibited by a change in internal pressure within the material. Such a change in pressure is generally linear with respect to the change in matric potential. (This can be compared to difficulties of using a volume change which will be generally non-linear with respect to matric potential).
In preference, the pressure applied is such that the externally applied pressure will be such that the physical pressure other than through the matric potential change will be approximately 300 kilopascals.
In a further form, in preference, there is a pressure transducer within the material such that any change in pressure within the material will be directly detected by the electrical transducer which in turn then can provide an electrical effect in accordance with the detected magnitude and/or change in pressure.
In another preferred arrangement, the material is subjected to a head of liquid by being separated from the material by a flexible membrane and such then that the head of liquid which can be open to atmosphere, will then exhibit a change in height in response to a change in pressure within the material which in turn depending upon the characteristic of the particular material, will expand or contract in accordance with the externally detected matric potential of the medium within which it is located.
In preference, such liquid to exhibit a change in pressure should be of high specific gravity so that liquid mercury can be used so that the upper most head of the mercury can then be calibrated in accordance with changes in expansion of the material which in turn are caused by a change in internal pressures in accordance with the balance of matric potential effected.
In such an example, the liquid itself provides the first pressure to establish a physical state of the material so that any externally detected matric potential will reduce the internal pressures within the material proportionately.
For instance, if the material is pressed to be 300 kilopascals, and it is put into hydraulic communication with soil having 100 kilopascals matric potential, then this will reduce the volume of the material to a 400 kilopascals volume.
Such a change can then be detected by detecting the volume change of the material for example by a simple sight glass.
In preference, the housing including or being totally comprised of a porous material has for its purpose to provide for a hydraulic communication which will allow the transfer of matric potential between the materials on respective sides of the porous material and to ensure a stable volume so that the volume change of the swellable material will be reliably transferred to the measuring means.
Baked and unglazed ceramic materials conventionally exhibit this characteristic and in experiments so far have shown to be appropriate.
Ceramic material is substantially rigid so that changes within the internal pressure of the material held with at least one part against the ceramic will not unduly deflect the ceramic so that the position of other portions of the material can be used to gauge the effect of the change in internal volume.
The ramifications of these steps and features are very high indeed.
Conventionally, a tensiometer is simply filled with water and when in a medium exhibiting more than 80 kilopascals suction, this will cause the water within the housing of the tensiometer to potentially break into a vacuum status dependent upon the vapor pressure of the water at the time and therefore be no longer useful to measure any different or extended suction range.
Growing plants mostly access water when held within a range of matric potential from 0 kilopascals to 300 kilopascals (they can less comfortably access water from soil at over 1,500 kpa) within the soil so that the one conventional technique of measuring soil suction is not useful outside of a small portion of the range of suction that should be able to be detected.
The discovery of this invention is that we need no longer be subject to the difficulties associated with the previously known simple tensiometer.
Accordingly this is because liquid preferably within an appropriate porous housing is held by a medium which, of itself, exhibits an internally compressive pressure and is exhibiting this effect by reason of molecular attraction within the structure of the material.
It will be readily appreciated that many different approaches can now be used to apply this concept to a number of various responders both for the purpose of effecting an output that of itself can be used to perform work by way of directly changing the open or closed status of a valve or in another form, have the pressure directly detected to effect either an electrical or visual output in response to such detected pressure changes in response to changes in the matric potential effect.
It is to be understood that what is being described is different from something which merely uses a material which will expand when wet and which will contract when dry.
The material that is to be used is something that can exhibit a structure which will provide a force with the effect that water will be held against an externally applied force and will give up that water with the extent of that force.
It has been found that a material which is a solid hydrogel can be used as one example, and in another example, a material which can absorb so much water as to be essentially fluid like, nonetheless can also exhibit the necessary characteristics.
If the material is to be solid, in preference, there are arrangements firstly to ensure that there is an effective hydraulic connection to the material from the porous barrier and further the solid material is arranged so that any pressure changes internally will result in a predictable expansion or contraction or change in pressure or in these all which can then be used to accurately assess the effective result.
In practice this has meant that the material can be in the form of thin disks which are flat and stacked one upon the other with sheets of interlocking fibrous material therebetween with solid plates such as metal, plastic or ceramic plates therebetween. Most of the transverse to the planar direction of the disk shape will expand or contract in response to the detected change in matric potential with the planar direction expansion being inhibited substantially to an extent that the volume is constrained howbeit with interleaving fibrous sheets.
The interleaving wicking material is arranged to extend to a side of the stack to be in contact with the material for matric potential transfer. In another approach, there is a solid material which is however divided with a number of slots which are filled with wicking material or at least material that will transfer liquid to provide a matric potential communication.